Radio devices for high-performance wireless systems may be built around a Gilbert-mixer architecture. A Gilbert-mixer is a frequency-translating circuit where the baseband (BB) signal is converted into a current, which current is then switched periodically by transistor switches in accordance with a local oscillator signal onto a radio-frequency (RF) load. However, radio transmitters based on such architectures (e.g., SAW-less radio transmitters) have a fundamental problem with out-of-band noise when operating in Frequency Division Duplex (FDD) mode which affects severely the operation of the radio receiver. Therefore, the noise generated by a radio transmitter in the frequency band of the receiver must be extremely low.
A high output noise in the frequency-band of the receiver in such Gilbert-mixer based radio architectures is mainly due to the modulated baseband noise. The baseband current in such mixer does not only carry the useful signal, but also higher-frequency noise components that may be difficult to filter out. A low-pass filter is thus needed, but such filter cannot be realized in the current-domain without using inductors, which are bulky and expensive. Therefore, a low-noise radio transmitter design is required.
Conventionally in transmitter devices, a low noise level (for example, a noise level of −183 dBm/Hz) is achieved by sufficient filtering in both the duplexer (for example, 50 dB) and in the Surface Acoustic Wave (SAW) filter of the transmitter. The SAW filter in the transmitter, however, limits the operation across multiple bands and is not tolerated anymore in a modern cost-effective solution. Therefore, the transmitter device itself must achieve a low out-of-band noise level in the order of −160 dBm/Hz.
Xin He, et. al. proposes in “A Low-Power, Low-EVM, SAW-Less WCDMA Transmitter Using Direct Quadrature Voltage Modulation,” IEEE Journal of Solid-State Circuits, vol. 44, no. 12, pp. 3448-3458, December 2009, an alternative transmitter architecture based on baseband voltage sampling instead of current switching. In this architecture, the four (quadrature and differential) baseband voltages are sampled consecutively onto the RF output capacitance with a 25% duty-cycled LO signal to effectively perform a frequency upconversion. The technique is similar to a passive mixer combined with a passive RC pole in the baseband chain to filter out out-of-band baseband noise before upconversion takes place. However, this architecture requires buffer circuits to drive the baseband inputs of the mixer. The buffer circuits thus need to drive rather low impedance levels (order of magnitude for R and C are 100 ohm and 100 pF), with a very large signal swing (to achieve sufficient signal-to-noise ratio) and with very stringent linearity requirements. In addition, as this implementation is very analog-intensive, it becomes more and more difficult to be implemented in newer-generation digital CMOS technologies.
Alternative radio transmitters were proposed employing a Direct Digital RF Modulator (DDRM). DDRM is based on a technique that merges a digital-to-analog converter (DAC) with an upconverter (mixer) in one single block to realize a radio frequency DAC (RFDAC). As such, however, there is no baseband signal in the analog domain. Thus, the traditional reconstruction filter after digital to analog conversion that typically attenuates the quantization noise and removes the aliases (i.e. the replicas of the digital signal around the sampling frequency and its harmonics) cannot be used. To offer sufficient filtering, high oversampling ratios (OSR) must be used to move these aliases to far-away locations and to exploit the filtering effect of the DAC sin(x)/x (sin c) response. DDRM-based radio architectures can achieve a good in-band signal quality, but fail to achieve a sufficiently low out-of-band noise level. Moreover, both the in-band and the out-of-band noise level can rise above the levels required by the telecommunication specifications, and especially for cellular systems.
Yoo et. al. disclose in “A Class-G Switched-Capacitor RF Power Amplifier,” IEEE Journal of Solid-State Circuits, Vol. 48, No. 5, pp. 1212-1224, May 2013, an alternative technique based on a switched-capacitor power amplifier (SCPA). The SCPA consists on an array of capacitors, a number of which (determined by the desired output amplitude) are switched at RF frequency between supply voltage and ground, thus generating an RF waveform. This system is capable of generating an RF output of reasonable accuracy, but generates a large amount of output noise both in the adjacent and far-out channels, which make it unusable in a practical system.
There is thus a need for novel transmitter architectures that are more digital-intensive and power efficient, that can be implemented in a nanoscale CMOS process using a small area.